This invention is in the field of stable latexes of chloroprene homopolymers and copolymers, prepared by polymerization of the corresponding monomers in the presence of polyvinyl alcohols.
In the prior art, difficulty has been encountered in making, by aqueous emulsion polymerization, aqueous latexes of polychloroprene which are stable against coagulation upon the addition of electrolytes and other additives commonly included in latex formulations. Zinc oxide, for instance, is a generally used constituent of compounded latexes. The addition of dry zinc oxide to chloroprene polymer latexes prepared by aqueous, alkaline polymerization of the corresponding monomer results in coagulation of the latex.
Such latex coagulation is not to be confused with the "layering" which occurs upon storage of solvent cements made by processes described in Kadowaki et al. U.S. Pat. No. 3,703,568. According to this patent, if a masticated polychloroprene, a modified phenol resin, and a metal oxide such as zinc or magnesium oxide are mixed to form a homogeneous compound and separately an organic solvent is mixed and reacted with a modified phenol resin and a metal oxide, and these two mixtures are mixed, a homogeneous adhesive composition is obtained. However, if this adhesive is stored for several months, it separates into an upper layer containing the solvent and a lower layer containing the metal oxide.
The Kadowaki et al. process avoids this layering of metal oxide by using in the adhesive a modified rubber obtained by adding a specified amount of polyvinyl alcohol (PVA) in the production step of the chloroprene rubber. The PVA may be added, if desired, to the purified polychloroprene latex -- that is, to the latex from which unpolymerized chloroprene monomer has been purged. Thus, in the Kadowaki patent the PVA appears to perform the same function as it does in U.S. Pat. No. 1,629,161 to Hermann et al. -- namely, only to provide a protective colloid to maintain the metal oxides in suspension.
Latex adhesives for making paper-metal film laminates, in order to have optimum utility, have heretofore required the incorporation of a protein such as casein into the latex. This entails extra expense for the casein and adds another component to an already complicated composition. There is a need in the art for a paper-metal foil adhesive not requiring casein or other protein as a component.